High definition television (HDTV) receivers receive signals,. vestigial sideband (VSB) signals, from a portion of the radio frequency spectrum that is also occupied by conventional television signals. These conventional television signals are generally referred to as the National Television Standards Committee (NTSC) signals. To accurately demodulate a VSB signal, HDTV receivers require an NTSC signal rejection filter to be used in the demodulator of the receiver. The rejection filter is intended to suppress the NTSC signals such that reception and demodulation of the VSB signal is not affected by the presence of an NTSC signal. The use of an NTSC rejection filter is specified in the xe2x80x9cGuide To The Use Of The ATSC Digital Television Standardxe2x80x9d Document A/54, Advanced Television Systems Committee, Apr. 12, 1995 and incorporated herein by reference. However, it is well known in the art that the NTSC rejection filter suppresses the VSB signal by approximately 3 dB. To avoid the VSB signal suppression caused by the NTSC rejection filter, the ATSC standard recommends switching the NTSC rejection filter into the signal path of the HDTV receiver only when necessary, i.e., only when an NTSC signal is detected in the VSB channel.
The ATSC standard recommends using a pair of energy detectors, where one detector is located before the NTSC rejection filter and one detector is located after the NTSC rejection filter. These detectors measure the signal-to-interference plus noise ratio of the received signal. The signal energy is measured by each detector and compared such that the signal path with the largest signal-to-noise ratio (lowest interference energy) is selected, i.e., either the path with the NTSC rejection filter or no filter is selected.
When the NTSC signal level is near to the noise level of the received signal, the standard detection technique may erroneously decide to include (or not include) the NTSC rejection filter into the signal path. Whenever the NTSC rejection filter is incorrectly utilized, the VSB signal is not optimally processed by the HDTV receiver and errors result in the decoded VSB signal data.
Therefore, a need exists in the art for an HDTV receiver that accurately detects the presence of a co-channel interference signal by directly measuring the error rate of the HDTV receiver with and without the co-channel interference rejection filter and selectively utilizes the co-channel interference rejection filter depending upon the measured error rate.
The disadvantages heretofore associated with the prior art high definition television (HDTV) receivers are overcome by the present invention of a method and apparatus for detecting the presence of a co-channel interference signal (e.g., an NTSC signal) and, in response to detecting such an interference signal, utilizing a co-channel interference rejection filter. Specifically, the present invention monitors the status information generated by the error detecting/correcting circuitry of a signal receiver such as an HDTV receiver. A counter is used to determine an error rate. While this first error rate is being determined, the co-channel interference rejection filter is not used within the signal path of the receiver. The filter is then switched into the signal path of the receiver and a second error rate is determined. The first and second error rates are then compared and if the first error rate is the same or lower than the second error rate, the co-channel interference rejection filter is not utilized. Otherwise, the co-channel interference rejection filter is used in signal path of the receiver to remove an interference signal that is interfering with the proper demodulation of the received signal.
More specifically, in an HDTV receiver application, a counter is used to count the number of errors that have been detected/corrected by a Reed-Solomon decoder over a predefined period of time. While this first count is being accumulated, an NTSC rejection filter is not used within the signal path of the HDTV receiver, i.e., a mulitplexer switches the NTSC rejection filter out of the signal path. The first count (first error rate) is stored in memory. The NTSC rejection filter is then switched into the signal path such that the received signal is filtered by the NTSC rejection filter. The status information of the Reed-Solomon decoder is again monitored and a second error rate is accumulated. The first and second error rates are then compared and if the first error rate is the same or lower than the second error rate, then the NTSC rejection filter should not be utilized. If the second error rate is lower than the first error rate, then the NTSC rejection filter should be used by the receiver to remove an NTSC signal that is interfering with the proper demodulation of the received signal. In this manner, the NTSC rejection filter is selectively utilized such that the operation of the HDTV receiver is optimized. Additionally, at a time in the future when all NTSC broadcast stations no longer are transmitting NTSC signals, the HDTV receiver will optimally demodulate HDTV signals without interference by the NTSC rejection filter.
The present invention provides a number of advantages of the prior art. First, the present invention bases its decision to utilize the NTSC rejection filter or not upon the actual error rates of received signals. As such, when the NTSC signal level is near the noise level, the error rate should not be impacted and the present invention correctly decides whether to use the NTSC rejection filter. Another advantage of the present invention is its substantial cost savings. The prior art decision circuitry uses a complex measurement and computation technique requiring many costly multiply and divide operations that require a substantial amount of silicon area when the circuit is implemented in hardware. By relying upon the error indicator output of the Reed-Solomon decoder, the present invention uses a minimal amount of additional hardware to accomplish the error rate computation and comparison processes. As such, the silicon area needed to implement the invention and its power requirements are substantially less than that of the prior art circuitry.